(a) Field of the Invention:
The present invention relates to an image forming optical system and particularly to an image forming optical system including an optical low-pass filter.
(b) Description of the Prior Art:
In an optical instrument carrying out the spatial sampling of an object image for photographing like a television camera using a CCD (charge coupled device), as an image pickup device, equipped with a color encoding filter such as a color mosaic filter and a color striped filter, a false signal is inevitably generated, termed aliasing or moire, due to beats caused by the spatial frequency and sampling frequency of the object image. Since the sampling of the object image is carried out with respect to various frequencies depending on an array of respective color elements of the color mosaic filter and an array of pixel elements of the CCD, the false signal is also produced in various frequencies. In order to diminish the generation of such a false signal, an optical low-pass filter has been provided in an image forming optical system of the optical instrument of the type. In general, the optical low-pass filter is constructed so that a plurality of birefringent plates such as quartz plates are properly combined with each other and the response of the image forming optical system is reduced to zero in respective sampling frequencies, thereby suppressing spatial frequency spectra of the object image in the vicinity of respective sampling frequencies.
Although the filter constructed as mentioned above fails usually to bring about problems in particular, a problem arises that if a frequency component similar to the sampling frequency among the spatial frequency spectra of the object image is considerably large, it is not completely suppressed and consequently the generation of the false signal cannot be prevented.
Specifically, such an instance occurs in the case where the object image derived from a fiberscope is photographed with a TV camera and will be described in the following. FIG. 1 depicts typically a structure for photographing the image derived from the fiberscope with a TV camera. Reference numeral 1 represents a fiberscope, which is provided with an objective lens unit 2 in a distal end portion and an image guide fiber bundle 3 in an inside portion. An entrance end face of the image guide fiber bundle 3 is arranged at an image forming position established by the objective lens unit 2 and an exit end face is located in an eyepiece section 4. Further, an eyepiece 5 is provided in the rear thereof Reference numeral 6 represents an adaptor for mounting a TV camera 7 to the eyepiece section 4 of the fiberscope in which an image forming lens 8 is housed. In the TV camera 7 are arranged an optical low-pass filter 9 and a CCD 11 having a color mosaic filter 10 on its entrance surface. It follows from this that an object 12 is formed as an image on the entrance end face of the image guide fiber bundle 3 through the objective lens unit 2 and then the image transmitted onto the exit end face is reformed by means of the eyepiece 5 and the image forming lens 8 through the optical low-pass filter 9 on the CCD 11. Here, the exit end face of the image guide fiber bundle 3 is formed from light spots distributed in a dot shape as shown in FIG. 2A because light is transmitted only by core portions of individual fibers Therefore, the object image appearing on the exit end face, for example, the distribution of light intensity at each spot position lying along a line A has a fine structure similar to the case where amplitude modulation (AM) of a fundamental wave depending on the repetition of cores as shown in FIG. 2B is produced by the intensity distribution of light of the object. As a result, a very large spectrum component exists in the spatial frequency corresponding to the fundamental wave and a common optical low-pass filter having a point where the response is a zero in the sampling frequency cannot completely suppress the large spectrum component, so that the false signal will remain.
The same phenomenon occurs also in the case where a lecture scope is utilized for observation. FIG. 3 shows typically a structure for observing the image transmitted by the fiberscope through the lecture scope, and the fiberscope per se is the same as in FIG. 1. The lecture scope 21 mounted to the eyepiece section houses a beam splitter 22, an image forming lens 23, an optical low-pass filter 24, an image guide fiber bundle 25, and an eyepiece 26 and is adapted to form the object image transmitted onto the exit end face of the image guide fiber bundle 3, through the eyepiece 5 and the image forming lens 23, onto an entrance end face of the image guide fiber bundle 25 and to transmit the image onto an exit end face of the image guide fiber bundle 25, thus observing it through the eyepiece 26. In such a case, since the image guide fiber bundle 25 transmits only light received through the core portions of individual fibers, it follows that the object image is sampled in the spatial frequency depending on the repetition of cores. Also, the exit end face of the image guide fiber bundle 3 on which an object image is formed has a large spectrum component in the repetition frequency of the core, with the result that if such an optical low-pass filter that the response is reduced to zero in the sampling frequency of the image guide fiber bundle 25 is employed, the moire remains likewise.